PA-28 instructor reported a left main gear collapse on landing during a training flight. The aircraft came to a stop on the runway.

Narrative

Student was conducting final flight with instructor before checkride. We departed from ZZZ to the west to work on maneuvers. Upon completion of maneuvers; we went back to ZZZ to work on the landing portion of the 'mock checkride'. I had asked the student to demonstrate a soft-field landing as the first into ZZZ. The landing was not within limitations because the nose gear was not kept up enough to distinguish it from a normal landing. On the go; I asked the student to perform another soft field landing so I could re-evaluate and provide additional training to help perform the maneuver within standards. As we approached the runway for the second landing; I carefully instructed the student to level off over the runway to enter ground effect. I instructed the student to pitch up slightly with every descent that occurred to ensure the nose wheel stayed up on touchdown. As our mains touched the ground I noticed the aircraft beginning to steer to the left. At first; I thought it was the student not maintaining centerline so I instructed him to do so. As we began to steer further from the centerline and noticed it was due to the aircraft inability and not the student; I demanded the controls and tried to recover. Upon recovery; the left main landing gear collapsed and the aircraft began to skid. I quickly informed the tower we had an issue on the runway and shut down the aircraft.